a. Field of the Invention
The present invention relates to a carburetor for an engine in an automotive vehicle such as a motorcycle, and the like, and, more particularly, it is concerned with an improvement in a variable venturi type carburetor for such engine, in which a piston type throttle valve serving also as a variable venturi is opened and closed by a negative pressure to be created at the time of air-intake into the engine so as to automatically vary the diameter of the venturi.
B. Description of Prior Art
For this kind of engine carburetor, there has so far been known such a construction that, by turning the handle grip of the motorcycle to operate the throttle, a control lever for opening and closing the throttle valve is caused to rotate to release the closed state of the piston type throttle valve, then a negative pressure created in the air intake passageway is introduced into a pressure receiving part provided on the upper part of the throttle valve through a passage formed in the throttle valve, by the negative pressure of which the throttle valve is opened, the degree of opening of the throttle valve being able to be restricted by its contact to the abovementioned control lever.
In the above-described carburetor, since the transition characteristic at the changing period of the degree of opening of the throttle valve is automatically determined by the negative pressure caused by the air intake of the engine, unless the closure state of the throttle valve is released, the number of revolutions of the engine increases augment the negative pressure at the air intake, the throttle valve is not raised, so that the venturi diameter remains small. And, following increase in the negative pressure at the air intake, the throttle valve is raised for an amount corresponding to the negative pressure value to increase the cross-sectional area of the venturi, and the throttle valve finally contacts the control lever for its opening and closing to maintain the stabilized degree of the throttle opening at that position.
Thus, the above-described throttle valve functions as the variable venturi in response to the negative pressure value at the air intake of the engine in its transitional state where the closure of the throttle valve is released, while it functions as the valve for regulating the maximum value of an amount of intake mixture air when it is at the stabilized position. Therefore, this type of variable venturi type carburetor has such advantage that it can stably and accurately increase speed without stoppage of the engine no matter how abrupt the speed acceleration in the engine might be.
On the other hand, when the time required for the speed acceleration is taken into consideration, no acceleration takes place by an abrupt accelerating operation, unless the negative pressure at the air intake of the engine increases. As the consequence, an additional time is required for this increase in the negative pressure until the speed acceleration is actually attained, on account of which there has remained a point of problem such that the feeling of the speed acceleration cannot be denied.
Besides the above, it has so far been ascertained that, in the variable venturi type carburetor wherein the throttle valve is opened and closed by a cable, when the grip handle is abruptly operated or turned to bring the throttle valve to its full open state, the speed acceleration is frequently failed to cause stoppage in the engine running, but, with respect to the speed accelerating operation for an intermediate degree of opening of the throttle valve, the number of revolutions of the engine gradually increases in pursuance of the speed accelerating operation with the consequence that there occurs no failure in the speed acceleration in the engine running. The reason for this is that, when the throttle valve abruptly opens to its full extent, the intake amount of air which is light in weight can immediately follow the variation in the degree of the valve opening to increase its quantity, while the fuel which is heavier in weight than air is delayed in time for its suction owing to difference in inertia between the intake air and the fuel flowing in the air intake passageway with the consequence that the mixing ratio between the air and the fuel becomes excessively thin and combustion of the mixed gas in the engine becomes unable to be continued. On the contrary, when the throttle valve is opened to its intermediate degree of opening, the increased portion in the amount of the intake air following the variation in the degree of opening of the throttle valve is smaller than that at its full open condition, so that the above-described tendency accrues.